In a code division multiple access high rate packet data (CDMA HRPD) system, signals are transmitted by using a signaling link protocol (SLP), and information interaction between an access network (AN) and an access terminal (AT) is realized by sending SLP message packets between the AN and the AT. Each SLP packet has a corresponding sequence number. The sequence number beginning from 0 is used for determining whether the SLP packet is a retransmitted message. When a reset process of SLP needs to be performed for the AT, the reset process of the SLP is accomplished by sending a Reset message and a ResetAck message between the AN and the AT. The Reset message and the ResetAck message also have the corresponding message sequence numbers, and the message sequence numbers of the Reset message and the ResetAck message in the same flow are identical.
As shown in FIG. 1, the main flow of the reset process of SLP is given below.
In Step 100, an AN initializes the local sequence number of the SLP packet for the AT (including sending message sequence number and receiving message sequence number).
In Step 101, the AN sends a Reset message to the AT.
In Step 102, the AT receives the Reset message, and then verifies the message sequence number of the Reset message.
In Step 103, when determining that the Reset message is valid, the AT feeds back a ResetAck message to the AN.
On the other hand, when determining that the Reset message is invalid, the AT discards the Reset message.
In Step 104, the AT initializes the local sequence number of the SLP packet.
In Step 105, the AN receives the ResetAck message, and verifies the message sequence number of the ResetAck message. For example, if the message sequence number of the ResetAck message is identical to the message sequence number of the Reset message delivered by the AN, the ResetAck message is considered to be valid.
In the above method, the valid check on the Reset message is performed by verifying the message sequence number of the Reset message, and the verification method is described below. Given that the sequence number occupies S bits, if the sequence number is N, [N+1, N+2S−1−1] is considered to be greater than N, and [N−5S−1, N−1] is smaller than N. When the AT receives the Reset message, if the message sequence number carried in the Reset message is greater than the message sequence number of the Reset message previously received by the AT, the Reset message is considered valid; otherwise, the message is considered invalid.
In the conventional CDMA HRPD system, the AT verifies the validation of the Reset message according to the message sequence number of the Reset message. Thereby, during a connected state handoff between the ANs, since the source AN cannot transmit the message sequence number of the Reset message to the target AN, the target AN cannot perform a reset process on the AT. Therefore, the sequence number of the SLP packet subsequently sent between the AT and the target AN is determined to be invalid, thus causing packet loss.
For example, if the Reset message sequence number of the AT on the source AN is 2 and the Reset message sequence number on the AT is also 2, after the AT is switched from the source AN to the target AN, since the target AN fails to receive the Reset message sequence number of the AT, the Reset message sequence number of the AT on the target AN is 1. When the target AN performs a reset process on the AT, the Reset message sequence number “1” is sent to the AT, and AT determines that “1” is smaller than the local Reset message sequence number “2,” so that a reset process will not be performed, that is, the local SLP data structure and the corresponding buffer will not be cleared and the sequence number of the local SLP packet will not return to zero. After the source AN sends the Reset message sequence number “1,” the local SLP data structure and the corresponding buffer of the AT and the sequence number of the SLP packet are initialized, so that the AT relevant information at two sides are inconsistent. When the target AN sends the SLP packet to the AT, since the sequence number of the SLP packet is smaller than the sequence number of the local SLP packet of the AT, the AT determines that the SLP packet subsequently sent by the target AN is invalid and then discards the SLP packet, thus causing SLP packet loss. Such a packet loss will trigger failures to receive calls from other ATs or to use subscribed application services, thereby causing inconveniences to subscribers.